We just finished our move from 10.1 to 10.3 (last week). For the most part it went pretty smooth. I did notice though that a couple of my python scripts started to crash; returning Error 000210 "Unable to output to location.. blah blah". I was able to replicate this issue inside of ArcMap using the Copy Features script from the toolbox. It will not trigger it when going to a Geodatabase, but if saving as a Shapefile it will return the same error.
A workaround for this was to use FeaturetoPoint instead. Hope this helps, I plan on submitting it to Esri later today.